Reducing magnetic field instabilities caused by oscillations of a mechanical cryo-cooler in magnetic resonance systems

ABSTRACT

Described here are systems and methods for mitigating or otherwise removing the effects of short-term magnetic field instabilities caused by oscillations of the cold head in a cryogen-free magnet system used for magnetic resonance systems, such as magnetic resonance imaging (“MRI”) systems, nuclear magnetic resonance (“NMR”) systems, or the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/071,774 filed Jul. 20, 2018, which represents the national stageentry of PCT International Application PCT/IB2016/051344, filed Mar. 9,2016. The contents of these applications are hereby incorporated byreference as set forth in their entirety herein.

BACKGROUND OF THE INVENTION

The field of the invention is systems and methods for magneticresonance. More particularly, the invention relates to systems andmethods for reducing magnetic field instabilities caused by oscillationsof a mechanical cryocooler in magnetic resonance systems.

Conduction cooled, cryogen-free magnet systems require the cold head(i.e., the source of cooling) to be placed much closer to the magnetwindings than in a typical magnet system containing a helium bath. Thisrequirement is because the heat is transferred via conduction alongcopper and increasing the distance of the cold head from the windingsreduces the cooling efficiency significantly. It is also desirable tohave the cold head close to the windings in order to reduce the overallsize of the system.

There are two types of cold head currently available: Gifford-McMahon(“GM”), which is the most common cold head currently used, and pulsetube. Within both there is a material called the regenerator, thismaterial is responsible for removing the last bits of heat to get downto less than 4 Kelvin (K). The regenerator material is typicallycomposed of an Erbium Nickel compound that becomes magnetized whenexposed to an external magnetic field. The amount of magnetization ofthe material (its magnetic moment) is dependent on both the magnitude ofthe applied magnetic field as well as its temperature.

In a GM cold head the regenerator material moves up and down at awell-defined frequency, both exposing itself to a variable magneticfield as well as changing temperature throughout the cooling cycle. In apulse tube cold head the regenerator material does not move, but itstemperature will still fluctuate. In both cases, the oscillations andtemperature fluctuations cause the regenerator material to act like atiny magnetic dipole with fluctuating magnitude. In currently availablemagnetic resonance systems, this generated magnetic dipole has littleeffect because the cold head is so far away from the imaging region;however, for cryogen-free magnets, this is not the case. The short-termfield instability caused by this effect results in image “ghosting.”

Thus, there remains a need to provide systems and methods for reducingor otherwise eliminating magnetic field instabilities caused byoscillations of a mechanical cryocooler in magnetic resonance systems.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a method for reducing magnetic field instability effectscaused by time-varying dipole fields generated by oscillations of a coldhead in a mechanical cryocooler in a magnetic resonance system. Themethod includes determining a spatial profile for the magnetic fieldinstabilities caused by time-varying dipole fields generated byoscillations of the cold head in the mechanical cryocooler. Then, with acomputer system and based in part on the determined spatial profile forthe magnetic field instabilities, control parameters associated at leastone corrective electromagnetic field are determined. This at least onecorrective electromagnetic field is determined such that when generated,it will reduce the magnetic field instabilities caused by thetime-varying dipole fields generated by oscillations of the cold head inthe mechanical cryocooler. A cycle of the mechanical cryocooler istracked and at a time point determined by the cycle of the mechanicalcryocooler, the at least one corrective electromagnetic field isgenerated, thereby reducing the magnetic field instabilities caused bythe time-varying dipole fields generated by oscillations of the coldhead in the mechanical cryocooler.

It is another aspect of the present invention to provide a passivecompensation coil for reducing magnetic field instabilities caused bytime-varying dipole fields oscillating at an oscillation frequency andgenerated by a mechanical cryocooler that forms a part of a magneticresonance system. The passive compensation coil generally includes atleast one conductive loop composed of a conductive material andpositioned proximate a cold head in the mechanical cryocooler, wherein athickness of the at least one conductive loop is at least one skip depthof the conductive material based on the oscillation frequency at whichthe time-varying dipole fields oscillate.

It is still another aspect of the invention to provide an active shieldfor reducing magnetic field instabilities caused by time-varying dipolefields generated by a mechanical cryocooler that forms a part of amagnetic resonance system. The active shield generally includes magnetwindings positioned proximate a cold head of the mechanical cryocoolerand driven by a controller to generate electromagnetic fields thatreduce a magnetic moment of regenerator materials in the cold head,thereby reducing the magnetic field instabilities caused by time-varyingdipole fields generated by the mechanical cryocooler.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings that form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example mechanical cryocooler with an actively drivencompensation coil for mitigating field instabilities caused bytime-varying dipole fields generated by the cold head.

FIG. 2 is a flowchart setting forth the steps of an example method forcontrolling an actively driven compensation coil to mitigate fieldinstabilities caused by time-varying dipole fields generated by the coldhead in a mechanical cryocooler.

FIG. 3 is a flowchart setting forth the steps of an example method forcontrolling the shim and/or gradient coils in a magnetic resonancesystem to mitigate field instabilities caused by time-varying dipolefields generated by the cold head in a mechanical cryocooler.

FIG. 4 depicts an example mechanical cryocooler with one or more passivecompensation coils for mitigating field instabilities caused bytime-varying dipole fields generated by the cold head.

FIG. 5 depicts an example mechanical cryocooler with active shieldingfor mitigating field instabilities caused by time-varying dipole fieldsgenerated by the cold head.

FIG. 6 depicts an example mechanical cryocooler with passive shieldingfor mitigating field instabilities caused by time-varying dipole fieldsgenerated by the cold head.

FIG. 7 is a block diagram of an example magnetic resonance imaging(“MRI”) system, which may incorporate a mechanical cryocooler.

DETAILED DESCRIPTION OF THE INVENTION

Described here are systems and methods for mitigating or otherwiseremoving the effects of short-term magnetic field instabilities causedby oscillations of the cold head in a cryogen-free magnet system usedfor magnetic resonance systems, such as magnetic resonance imaging(“MRI”) systems, nuclear magnetic resonance (“NMR”) systems, or thelike. In general, such cryogen-free magnet systems include asuperconducting magnet cooled by a mechanical cryocooler. The mechanicalcryocooler can be a Gifford-McMahon (“GM”) cryocooler or a pulse tubecryocooler. Further, pulse tube cryocoolers can include GM-type orStirling type pulse tube cryocoolers.

The field instabilities caused by a mechanical cryocooler are caused bya time-varying dipole magnetic field at the location of the cold head.In general, these field instabilities have a frequency of oscillation ofabout 1-1.2 Hz. The systems and methods of the present invention providetechniques for mitigating or otherwise eliminating the deleteriouseffects of the magnetic field instabilities caused by the time-varyingdipole field generated by the mechanical cryocooler.

In some embodiments, the field instability caused by a mechanicalcryocooler can be mitigated using an actively driven compensation coil.As shown in FIG. 1 , the actively driven compensation coil 10 can bepositioned proximate the cold head 12 of a mechanical cryocooler. Theactively driven compensation coil 10 is driven by a controller 14according to the methods described below. The actively drivencompensation coil 10 can include a specially designed electromagnet, asdescribed below.

As was stated above, the field instability caused by a mechanicalcryocooler is caused by a time-varying dipole magnetic field at thelocation of the cold head. The spatial pattern of the dipole magneticfield can be measured, or simulated, and an electromagnet can bedesigned based on those measurements or simulations to remove theeffects of the time-varying dipole magnetic field. The current suppliedto the electromagnet would be selected to oscillate at the samefrequency as the field instability, thereby generating a magnetic fieldthat would interfere with and mitigate the field instability. Thisimplementation would require tracking of the cycle of the cold head.

Referring now to FIG. 2 , a flowchart is illustrated as setting forththe steps of an example method for mitigating the field instabilitycaused by a mechanical cryocooler using an electromagnet that isspecifically designed or operated to provide active compensation tocancel or otherwise mitigate the field instability. The method includesproviding information related to the time-varying dipole field generatedby the mechanical cryocooler, as indicated at step 202. In someinstances, this information can be provided by measuring the dipolefield generated by the mechanical cryocooler in operation. In some otherinstances, this information can be provided by simulating the dipolefield that should be generated by the mechanical cryocooler inoperation. This information can be used to determine correctiveelectromagnetic fields that, when generated in the proximity of the coldhead, will mitigate or otherwise eliminate the magnetic fieldinstabilities caused by the time-varying dipole fields. In someinstances, control parameters that define the operation of anelectromagnet to generate the corrective electromagnetic fields aredetermined.

In any event, an electromagnet to mitigate the field instability causedby that dipole field is then designed based on the provided information,as indicated at step 204. The design can include the physicalconstruction of the electromagnet, or can include information, such asthe control parameters mentioned above, about how to operate aparticular electromagnet to generate the corrective electromagneticfields to mitigate the field instability. In either instance, it ispreferable to track the cycle of the cold head in the mechanicalcryocooler. Thus, the cycle of the mechanical cryocooler is tracked, asindicated at step 206. Tracking the cycle of the mechanical cryocooleris done so that operation of the electromagnet can be synchronized withthe dipole fields generated by the mechanical cryocooler, therebyincreasing the mitigation of the field instability.

As one example, the cycle of the cold head in the mechanical cryocoolercan be tracked by measuring the dipole field as a function of time andidentifying where in the stability cycle the cryocooler currently isoperating. This measurement can be performed, for example, using themagnetic resonance system itself, or with a Hall probe, or the like,that is operably coupled to the cold head. In this instance, theinformation can be relayed back to the controller for the activelydriven compensation coil and used to appropriately drive the coil.

As another example, the motor of the cold head pump can be tracked. Thecycle of the motor can then be correlated with the dipole field toprovide information about the cycle of the mechanical cryocooler as itrelates to the dipole field being generated at a particular point in thecycle. In this instance, the information about the pump motor cycle canbe relayed back to the controller for the actively driven compensationcoil and used to appropriately drive the coil.

By tracking the cycle of the mechanical cryocooler and providing theinformation about the dipole field generated by the cryocooler, theelectromagnet can be operated in a manner such that a field is generatedthat mitigates the field instability caused by the mechanicalcryocooler, as indicated at step 208. As mentioned above, theelectromagnet may be structurally designed to provide greater efficiencyin mitigating the field instability; however, in general theelectromagnet design can include information about the currents to beprovided to the electromagnet that will generate the desired fields tooffset the field instability. Thus, using the information provided andobtained, as discussed above, the electromagnet can be operated as anactive compensation coil specifically tailored to mitigate the fieldinstability effects of the mechanical cryocooler.

In some embodiments, the field instability caused by a mechanicalcryocooler can be mitigated using an active compensation technique usingimaging coils and digital processing. The spatial profile of themagnetic field produced by the regenerator material in the cryocoolerwill fall off roughly as a function of 1/r³, where r is distance fromthe cold head. The frequency of the field instability oscillations canbe tracked and a combination of electromagnets within the bore of themagnetic resonance system (e.g., gradient and shim coils) can be usedtogether with a demodulation frequency to reduce the effects of theoscillations.

As one example, adjustment of the demodulation frequency will remove themean of the oscillation, applying the y-gradient with an oscillatingfrequency will remove a linear function of field oscillation, andapplying the second order shim coils with an oscillating frequency willremove second order terms of the field instability. This combinedapproach removes most of the effect of the problem without the need foradditional hardware that is not already present on the magneticresonance system. It will be appreciated, however, that one or more ofthese mitigating features can be implemented alone or in combinationdepending on the type and severity of the field instability effects.

Referring now to FIG. 3 , a flowchart is illustrated as setting forththe steps of an example method for mitigating the field instabilitycaused by a mechanical cryocooler using the shim coils, gradient coils,or both, of a magnetic resonance system. The method includes providinginformation related to the time-varying dipole field generated by themechanical cryocooler, as indicated at step 302. In some instances, thisinformation can be provided by measuring the dipole field generated bythe mechanical cryocooler in operation. In some other instances, thisinformation can be provided by simulating the dipole field that shouldbe generated by the mechanical cryocooler in operation. Informationrelated to the time-varying dipole field can include the frequency,magnitude, or both of the field. This information can be used todetermine corrective electromagnetic fields that, when generated in theproximity of the cold head, will mitigate or otherwise eliminate themagnetic field instabilities caused by the time-varying dipole fields.In some instances, control parameters that define the operation of shimcoils, gradient coils, or both, to generate the correctiveelectromagnetic fields are determined.

It is preferable to track the cycle of the mechanical cryocooler to timethe application of electromagnetic fields generated by the shim coils,gradient coils, or both. Thus, the cycle of the mechanical cryocooler istracked, as indicated at step 304.

As one example, the cycle of the cold head in the mechanical cryocoolercan be tracked by measuring the dipole field as a function of time andidentifying where in the stability cycle the cryocooler currently isoperating. This measurement can be performed, for example, using themagnetic resonance system itself, or with a Hall probe, or the like,that is operably coupled to the cold head. In this instance, theinformation can be relayed back to the magnetic resonance system toappropriately drive the shim coils, gradient coils, or both.

As another example, the motor of the cold head pump can be tracked. Thecycle of the motor can then be correlated with the dipole field toprovide information about the cycle of the mechanical cryocooler as itrelates to the dipole field being generated at a particular point in thecycle. In this instance, the information about the pump motor cycle canbe relayed back to the magnetic resonance system to appropriately drivethe shim coils, gradient coils, or both.

By tracking the cycle of the mechanical cryocooler and providing theinformation about the dipole field generated by the cryocooler, themagnetic resonance system's shim coils, gradient coils, or both can beoperated in a manner such that one or more fields are generated thatmitigate the field instability caused by the mechanical cryocooler, asindicated at step 306. As mentioned above, adjustment of thedemodulation frequency will remove the mean of the oscillation, applyingthe y-gradient with an oscillating frequency will remove a linearfunction of field oscillation, and applying the second order shim coilswith an oscillating frequency will remove second order terms of thefield instability. Thus, using the information provided and obtained, asdiscussed above, the magnetic resonance system can be operated toactively compensate for the field instability effects of the mechanicalcryocooler.

In some embodiments, such as those illustrated in FIG. 4 , the fieldinstability caused by a mechanical cryocooler can be mitigated using oneor more passively conducting loops 16 positioned near the cold head 12.As one example, conducting loops of wire, or other conducting material,could be placed near the base of the cold head. As the magnetic dipolechanges in magnitude, a changing magnetic flux through the conductingmaterial will be generated, and this changing flux will result in aninduced current to oppose the flux change. The induced current will actto cancel the field produced by the dipole throughout space. Thetime-constant of the conducting loop (a function of its resistance andinductance) should preferably be long compared to the frequency ofoscillation, which is about 1-1.2 Hz. Thus, the loop should be composedof an extremely low resistance material. As one example, the loop can becomposed of thick cold copper.

For the passive conducting loops, the thickness of the conductor shouldbe selected based on the skin depth for the conductor material at adriving frequency around the frequency of oscillation (e.g., around 1Hz). For copper cooled below 7 K, the resistivity of the material is2×10⁻¹¹ Ωm. For a 1 Hz driving frequency, the skin depth of copper isabout 2.83 cm. Therefore, to reduce the magnetic field by about sixtypercent, the thickness, t, of the copper must be one skin depth thick.To completely eliminate the effect, the thickness, t, of the coppershould be at least three skin depths thick (e.g., about 8.5 cm).

In some embodiments, such as those illustrated in FIG. 5 , the fieldinstability caused by a mechanical cryocooler can be mitigated byactively shielding the cold head 12 with an active shielding 18positioned proximate the cold head 12. Because the magnetic moment ofthe material depends on the magnitude of the external field, if themagnetic moments of the materials in the cold head are reduced the fieldinstability effects will be reduced as well. The magnetic moment in thecold head materials can be reduced by shielding the magnetic fieldaround the cold head with an active electromagnet 18 controlled by acontroller 20. For instance, the active shielding can include designingthe magnet windings to include the need for a region of low magneticfield around the cold head. In general, it is contemplated that theactive shield can be designed based on the type of regenerator materialin the mechanical cryocooler, the geometry of the mechanical cryocooler,the local magnetic field in which the mechanical cryocooler will bepositioned, or combinations thereof. For instance, the design of thewindings in the active shielding 18, the current waveforms through theactive shielding 18, or both, are in general chosen based on theparticular system. As an example, if only the regenerator materialchanged, then the same design of windings in the active shield 18 couldbe used, but the current waveform provided to the active shielding 18could have a different amplitude depending on how the regeneratormaterial magnetizes.

In some other embodiments, such as those illustrated in FIG. 6 , thefield instability caused by a mechanical cryocooler can be mitigated bypassively shielding the cold head 12 with passive shielding 22. As oneexample, the passive shielding 22 can include a ferromagnetic materialpositioned around the cold head. As another example, the passiveshielding 22 can include superconducting loops placed around the coldhead, but not in series with the magnet windings. In this latterexample, the superconducting loops will act to maintain zero magneticflux through their center, but must be designed so that ramping to fieldwill not cause these superconducting loops to exceed their criticalcurrent density.

Referring particularly now to FIG. 7 , an example of a magneticresonance imaging (“MRI”) system 700 is illustrated. The MRI system 700includes an operator workstation 702, which will typically include adisplay 704; one or more input devices 706, such as a keyboard andmouse; and a processor 708. The processor 708 may include a commerciallyavailable programmable machine running a commercially availableoperating system. The operator workstation 702 provides the operatorinterface that enables scan prescriptions to be entered into the MRIsystem 700. In general, the operator workstation 702 may be coupled tofour servers: a pulse sequence server 710; a data acquisition server712; a data processing server 714; and a data store server 716. Theoperator workstation 702 and each server 710, 712, 714, and 716 areconnected to communicate with each other. For example, the servers 710,712, 714, and 716 may be connected via a communication system 740, whichmay include any suitable network connection, whether wired, wireless, ora combination of both. As an example, the communication system 740 mayinclude both proprietary or dedicated networks, as well as opennetworks, such as the internet.

The pulse sequence server 710 functions in response to instructionsdownloaded from the operator workstation 702 to operate a gradientsystem 718 and a radiofrequency (“RF”) system 720. Gradient waveformsnecessary to perform the prescribed scan are produced and applied to thegradient system 718, which excites gradient coils in an assembly 722 toproduce the magnetic field gradients G_(x), G_(y), and G_(z) used forposition encoding magnetic resonance signals. The gradient coil assembly722 forms part of a magnet assembly 724 that includes a polarizingmagnet 726 and a whole-body RF coil 728.

RF waveforms are applied by the RF system 720 to the RF coil 728, or aseparate local coil (not shown in FIG. 7 ), in order to perform theprescribed magnetic resonance pulse sequence. Responsive magneticresonance signals detected by the RF coil 728, or a separate local coil(not shown in FIG. 7 ), are received by the RF system 720, where theyare amplified, demodulated, filtered, and digitized under direction ofcommands produced by the pulse sequence server 710. The RF system 720includes an RF transmitter for producing a wide variety of RF pulsesused in MRI pulse sequences. The RF transmitter is responsive to thescan prescription and direction from the pulse sequence server 710 toproduce RF pulses of the desired frequency, phase, and pulse amplitudewaveform. The generated RF pulses may be applied to the whole-body RFcoil 728 or to one or more local coils or coil arrays (not shown in FIG.7 ).

The RF system 720 also includes one or more RF receiver channels. EachRF receiver channel includes an RF preamplifier that amplifies themagnetic resonance signal received by the coil 728 to which it isconnected, and a detector that detects and digitizes the I and Qquadrature components of the received magnetic resonance signal. Themagnitude of the received magnetic resonance signal may, therefore, bedetermined at any sampled point by the square root of the sum of thesquares of the I and Q components:M=√{square root over (I ² +Q ²)}  (1);

and the phase of the received magnetic resonance signal may also bedetermined according to the following relationship:

$\begin{matrix}{\varphi = {{\tan^{- 1}\left( \frac{Q}{I} \right)}.}} & (2)\end{matrix}$

The pulse sequence server 710 also optionally receives patient data froma physiological acquisition controller 730. By way of example, thephysiological acquisition controller 730 may receive signals from anumber of different sensors connected to the patient, such aselectrocardiograph (“ECG”) signals from electrodes, or respiratorysignals from a respiratory bellows or other respiratory monitoringdevice. Such signals are typically used by the pulse sequence server 710to synchronize, or “gate,” the performance of the scan with thesubject's heart beat or respiration.

The pulse sequence server 710 also connects to a scan room interfacecircuit 732 that receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 732 that a patient positioning system734 receives commands to move the patient to desired positions duringthe scan.

The digitized magnetic resonance signal samples produced by the RFsystem 720 are received by the data acquisition server 712. The dataacquisition server 712 operates in response to instructions downloadedfrom the operator workstation 702 to receive the real-time magneticresonance data and provide buffer storage, such that no data is lost bydata overrun. In some scans, the data acquisition server 712 does littlemore than pass the acquired magnetic resonance data to the dataprocessor server 714. However, in scans that require information derivedfrom acquired magnetic resonance data to control the further performanceof the scan, the data acquisition server 712 is programmed to producesuch information and convey it to the pulse sequence server 710. Forexample, during prescans, magnetic resonance data is acquired and usedto calibrate the pulse sequence performed by the pulse sequence server710. As another example, navigator signals may be acquired and used toadjust the operating parameters of the RF system 720 or the gradientsystem 718, or to control the view order in which k-space is sampled. Instill another example, the data acquisition server 712 may also beemployed to process magnetic resonance signals used to detect thearrival of a contrast agent in a magnetic resonance angiography (“MRA”)scan. By way of example, the data acquisition server 712 acquiresmagnetic resonance data and processes it in real-time to produceinformation that is used to control the scan.

The data processing server 714 receives magnetic resonance data from thedata acquisition server 712 and processes it in accordance withinstructions downloaded from the operator workstation 702. Suchprocessing may, for example, include one or more of the following:reconstructing two-dimensional or three-dimensional images by performinga Fourier transformation of raw k-space data; performing other imagereconstruction algorithms, such as iterative or backprojectionreconstruction algorithms; applying filters to raw k-space data or toreconstructed images; generating functional magnetic resonance images;calculating motion or flow images; and so on.

Images reconstructed by the data processing server 714 are conveyed backto the operator workstation 702 where they are stored. Real-time imagesare stored in a data base memory cache (not shown in FIG. 7 ), fromwhich they may be output to operator display 702 or a display 736 thatis located near the magnet assembly 724 for use by attending physicians.Batch mode images or selected real time images are stored in a hostdatabase on disc storage 738. When such images have been reconstructedand transferred to storage, the data processing server 714 notifies thedata store server 716 on the operator workstation 702. The operatorworkstation 702 may be used by an operator to archive the images,produce films, or send the images via a network to other facilities.

The MRI system 700 may also include one or more networked workstations742. By way of example, a networked workstation 742 may include adisplay 744; one or more input devices 746, such as a keyboard andmouse; and a processor 748. The networked workstation 742 may be locatedwithin the same facility as the operator workstation 702, or in adifferent facility, such as a different healthcare institution orclinic.

The networked workstation 742, whether within the same facility or in adifferent facility as the operator workstation 702, may gain remoteaccess to the data processing server 714 or data store server 716 viathe communication system 740. Accordingly, multiple networkedworkstations 742 may have access to the data processing server 714 andthe data store server 716. In this manner, magnetic resonance data,reconstructed images, or other data may be exchanged between the dataprocessing server 714 or the data store server 716 and the networkedworkstations 742, such that the data or images may be remotely processedby a networked workstation 742. This data may be exchanged in anysuitable format, such as in accordance with the transmission controlprotocol (“TCP”), the internet protocol (“IP”), or other known orsuitable protocols.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

The invention claimed is:
 1. An active shield for reducing magneticfield instabilities caused by time-varying dipole fields generated by amechanical cryocooler that forms a part of a magnetic resonance system,comprising: magnet windings positioned proximate a cold head of themechanical cryocooler, wherein the magnet windings form a polarizingmagnet coil of the magnetic resonance system; and a controller incommunication with the polarizing magnet coil and configured to drivethe magnet windings of the polarizing magnet coil to generate a reducedmagnetic field proximate the cold head such that a magnetic moment ofregenerator materials in the cold head is reduced, thereby reducing themagnetic field instabilities caused by time-varying dipole fieldsgenerated by the mechanical cryocooler.
 2. The active shield as recitedin claim 1, wherein the magnet windings are driven by the controllerbased on current waveforms that are selected at least in part based onthe magnetic moment of the regenerator materials.
 3. The active shieldas recited in claim 1, wherein a geometry of the magnet windings isdesigned based at least in part on at least one of the magnetic momentof the regenerator materials or a magnetic field strength of themagnetic resonance system.
 4. The active shield as recited in claim 1,wherein the magnet windings are at least one of superconducting orresistive.
 5. The active shield as recited in claim 1, wherein themechanical cryocooler is one of a Gifford-McMahon cryocooler or a pulsetube cryocooler.